Cable used in the telecommunications industry, such as in telephone systems, generally requires a waterproofing filling material in the cable to protect the cable from water entry. This is true whether the cable is buried beneath the ground or laid under water. It is also sometimes required in aerial applications.
Attempts to waterproof cable such as buried cable began nearly 100 years ago and were unsuccessful in a practical sense until the introduction of plastic insulated cable during the 1950's. Specially sheathed cables, with dual plastic coatings encasing an aluminum shield, have been used successfully and are still being buried in dry environments. Pressurized cable also contends successfully with water problems. However, both of these approaches are deficient, the former leaves the cable vulnerable and the latter is expensive to maintain. Since 1970, large quantities of cable have been filled with waterproofing compounds. This approach followed the recognition that in plastic insulated cable the localized intrusion of water into the cable sheath is not in itself a serious problem. Disruption of deterioration of service occurs when long lengths of cable become flooded. Flooding occurs because water that penetrates into a localized opening in the cable sheath is free to channel as far as gravity allows, often hundreds of feet. Not only does this upset the capacitance balance of the transmission lines, but it introduces more potential corrosion sites in proportion to the length of wire that is wetted. Corrosion typically occurs slowly, but the useful life of water soaked wires is obviously shorter than that of dry wires.
A solution that has been widely adopted is to fill the voids in the cable with a water insoluble filling material that simply plugs the cable to channeling water. However, though the physical function of the cable filling material is straightforward, the choice of the material is not. Among the many considerations that are important for materials used in this application are the hydrophobic nature of the material, stability on aging, low temperature properties, flow characteristics at elevated temperatures, processing characteristics, handling characteristics, dielectric properties, toxicity and cost.
Materials that satisfy most of these criteria, and which have been used widely in this application, are described in U.S. Pat. Nos. 3,607,487 and 3,717,716 issued Sept. 21, 1971 and Feb. 20, 1973, respectively. These materials are essentially a petroleum jelly mixed with a polymer, usually polyethylene, to impart consistency and prevent flowing at warm temperatures.
Similar hydrophobic filling materials have been proposed for filling splice closures. For example, U.S. Pat. No. 3,879,575 issued Apr. 22, 1975 describes a mixture of a low viscosity oil, gelled by a styrene-isoprene-styrene copolymer, again with a polyethylene wax added to impart consistency and reduce slump.
More recently, an improvement over the petroleum jelly-polyethylene wax cable filling material has been disclosed in U.S. Pat. No. 4,259,540 issued Mar. 31, 1981. This patent discloses a material which overcomes the objectionable handling characteristics of the petroleum jelly-polyethylene cable filling material. For example, since installation and maintenance of cables often requires the cable to be spliced, such splicing generally requires the isolation and removal of filling material from individual wires or optical fibers in the splice region where the cables are filled with the petroleum jelly material. Otherwise, an oily interface may form between the wire and the polyurethane material subsequently used to encapsulate (waterproof) the splice. This oily interface can serve as a path for water entry into the splice. This results in soiling hands, equipment and clothing. Moreover, removing just sufficient material to effect the splice is time consuming and the task is generally undesirable. Further, handling low temperatures is significantly more difficult, necessitating on occasion, use of a torch to preheat the cable or the use of solvents to soften the encapsulated core. The improved material described in U.S. Pat. No. 4,259,540 overcomes the aforementioned objections to the cable filled with the petroleum jelly-polyethyene material. The improved material according to the patent is a mixture of a napthenic or paraffinic oil having specific characteristics, a styrene-ethylene butylene-styrene (S-EB-S) triblock copolymer having a styrene-rubber ratio of from about 0.2 to 0.5 and polyethylene having a softening point of 110.degree. C. to 130.degree. C.
It should be noted that the term styrene-rubber ratio, when used herein, refers to the weight ratio of the styrene block to the rubber block in the copolymer. Further, whenever the term S-EB-S is employed, it refers to a triblock copolymer whereas the term S-EB refers to a diblock copolymer.
While the cable in accordance with U.S. Pat. No. 4,259,540 is excellent for use in underground applications, it has certain shortcomings with respect to use for aerial applications. These shortcomings are due generally to compound flow-out of the cable especially where solar heating (up to 80.degree.C.) is a factor. Also, at elevated temperatures, the mutual capacitance between electrical conductors of a typical copper wire cable increases due to migration of the oil component into the cell structure of the conductor insulation which generally consists of foamed polyethylene. While U.K. Patent Application GB No. 2 092 176 A teaches that such cell filling can be prevented by the addition of a polybutene oil, and it is generally known that high temperature flow can be attained by increasing the polyethylene wax level present in such compositions, unfortunately, the addition of polybutene oil and an increase in polyethylene wax level both lead to an increase processing viscosity. High viscosity makes it difficult to penetrate and fill the core of large pair count cable. As a general rule, filling should be done at no higher than about 110.degree. C. to prevent damage of the foamed polyethylene insulation and should have a viscosity of less than about 60 centipoise at that temperature to effect filling of large pair sized cable.
Attempts to lower the viscosity by reducing the rubber content of the S-EB-S copolymer have been unsuccessful because such compounds do not properly gel the oil. That is, a minimum level of S-EB-S, of about 5.0 percent by weight is required. Even at a 5.0 percent by weight S-EB-S level, some slight oil separation (syneresis) is often seen as a result of these problems. In order to control this syneresis, U.S. Pat. No. 4,259,540 teaches that a syneresis inhibitor may be added. Such inhibitors are not necessary with the filling formulations of this invention. Filled cables intended for aerial use have used a blend of petroleum jelly and polyethylene wax. Such cables exhibit the drawbacks of the buried petroleum jelly cables previously discussed.
Applicant has now discovered a filling composition which is a modification of the composition described in U.S. Pat. No. 4,259,540 which is included herein by reference, and which overcomes the problems stated above for aerial use, but yet retains all of the other properties which make the cable suitable for use as buried cable. Consequently, cable using the filling compound as described herein is not only suitable for use as buried cable, but is also suitable for aerial use as well.